This invention relates to high intensity discharge (HID) lamps and more particularly to sintering aids for use with the electron emissive material used in the electrodes of such lamps. High pressure sodium-mercury vapor lamps have in the past utilized as electron emissive material a mixture of several oxide phases comprising thorium dioxide, barium thorate, dibarium calcium tungstate and barium oxide. This mixture of oxide phases was quite sensitive to the atmospheric contaminants with a result that even a brief exposure to air resulted in a relatively large absorption of water and carbon dioxide by the emission material. Such contaminatnts were rather difficult to remove. Silica (SiO.sub.2) was an effective sintering aid for the foregoing emission material.
In U.S. Pat No. 3,708,710 dated Jan. 2, 1973, there is disclosed a high intensity discharge lamp utilizing an electron emitting material of dibarium calcium tungstate (Ba.sub.2 CaWO.sub.6) that has the properties of providing copious electron emission and resistance to vaporization and ion bombardment. This material is particularly suitable as emissive material for electrodes used in high pressure sodium vapor lamps where electron emission is necessary above 1000.degree. C. and electron emission material evaporation rates must be kept low.
In U.S. Pat. No. 4,052,634 dated Oct. 4, 1977, there is disclosed a high-pressure gas discharge lamp having an electrode with an electron emitting material which contains an alkaline earth metal and at least one of the metals tungsten and molybdenum, and is characterized in that the electron emitting material consists mainly of at least one oxidic compound containing at least one of the rare earth metal oxides, alkaline earth metal oxide in a quantity of 0.66 to 4 mole per mole of rare earth metal oxide and at least one of the oxides of tungsten and molybdenum in a quantity of 0.25 to 0.40 mole per mole of alkaline earth metal oxide, the alkaline earth metal oxide consisting of at least 25 mole percent of barium oxide.
In U.S. Pat. No. 4,123,685 dated Oct. 31, 1978, there is disclosed a high intensity discharge lamp utilizing an electron emissive material consisting essentially of a solid solution of dibarium calcium tungstate (Ba.sub.2 CaWO.sub.6) and dibarium calcium molybdate (Ba.sub.2 CaMoO.sub.6) wherein the molar ratio of the tungstate to molybdate falls within the range of from 9:1 to 1:9. The emissive properties of the electrode are especially well suited for use in high pressure sodium mercury lamps because the vapor pressure of the emission material is low resulting in low evaporation of the emissive material.
In U.S. Pat. No. 4,152,619 dated May 1, 1979, there is disclosed a high intensity discharge lamp with an electron emissive material portion of the lamp electrodes consisting of M.sub.3 M'.sub.2 M"O.sub.9 wherein M is an alkaline earth metal and at least principally comprises barium; M' is yttrium, a lanthanide series rare earth metal, or any mixtures thereof; and M" is tungsten, molybdenum, or mixtures thereof. The specified material is stable, highly electron emissive and has a low vapor pressure.
In U.S. Pat. No. 4,152,620 dated May 1, 1979, there is disclosed a sintering aid which improves the sintering of emission materials in high pressure discharge lamps. The sintering aid is a eutectic composition of at least one of barium oxide-tungsten oxide (BaO-WO.sub.3), calcium oxide-tungsten oxide (CaO-WO.sub.3), or strontium oxide-tungsten oxide (SrO-WO.sub.3) and is mixed with the emission materials in quantities of about 2 to 50 weight percent sintering aid. This sintering aid results in a significantly improved electrode structure and essentially elminates any problems of dusting during handling of finished electrodes and finished lamps which can cause contamination of the arc tube, additionally it introduces no foreign elements into the emission material described in U.S. Pat. No. 4,123,685 and 4,152,619.
In U.S. Pat. No. 4,321,503 dated Mar. 23, 1982, there is disclosed a high intensity discharge lamp where the electron emissive material portion of the lamp electrodes is Ba.sub.3 CaM.sub.2 O.sub.9 wherein M is niobium, tantalum, or any combination thereof. Such electrode material is highly emissive, refractory, and essentially non-reactive with water.